The present invention relates to a corner windbox air compartment of a fossil fuel-fired furnace equipped with a tangential firing system and an exit assembly for such an air compartment.
It is known that a staged combustion approach can improve the reduction of NO.sub.X in a fossil fuel-fired furnace such as, for example, a furnace in which pulverized coal is fired. Such a staged combustion approach may include reducing the quantity of air introduced into a main burner region of the furnace, which is a region in which the fuel such as the pulverized coal is injected, and instead introducing greater quantities of air above the main burner zone.
Over the years, there have been different approaches pursued in the prior art insofar as concerns addressing the need to limit emissions of the NO.sub.X that is created as a consequence of the combustion of fossil fuels in furnaces. The focus of one such approach has been on developing so-called low NO.sub.X firing systems suitable for employment in fossil fuel-fired furnaces. U.S. Pat. No. 5,020,454 entitled "Clustered Concentric Tangential Firing System", which issued on Jun. 4, 1991 and which is assigned to the same assignee as the present patent application discloses an example of one such low NO.sub.X firing system. In accordance with the teachings of U.S. Pat. No. 5,020,454, a clustered concentric tangential firing system is provided that includes a windbox, a first cluster of fuel nozzles mounted in the windbox and operative for injecting clustered fuel into the furnace so as to create a first fuel-rich zone therewithin, a second cluster of fuel nozzles mounted in the windbox and operative for injecting clustered fuel into the furnace so as to create a second fuel-rich zone therewithin, an offset air nozzle mounted in the windbox and operative for injecting offset air into the furnace such that the offset air is directed away from the clustered fuel injected into the furnace and towards the walls of the furnace, a close-coupled overfire air nozzle mounted in the windbox and operative for injecting close-coupled overfire air into the furnace, and a separated overfire air nozzle mounted in the windbox and operative for injecting separated overfire air into the furnace.
Another example of such a low NO.sub.X firing system is that which forms the subject matter of U.S. Pat. No. 5,315,939 entitled "Integrated Low NO.sub.X Tangential Firing System", which issued on May 31, 1994 and which is assigned to the same assignee as the present patent application. In accordance with the teachings of U.S. Pat. No. 5,315,939, an integrated low NO.sub.X tangential firing system is provided that includes pulverized solid fuel supply means, flame attachment pulverized solid fuel nozzle tips, concentric firing nozzles, close-coupled overfire air, and multi-staged separate overfire air and when employed with a pulverized solid fuel-fired furnace is capable of limiting NO.sub.X emissions therefrom to less than 0.15 lb./million BTU while yet maintaining carbon-in-flyash to less than 5% and CO emissions to less than 50 ppm.
Both of the tangentially fired furnaces disclosed in the two afore-mentioned references capitalize on the knowledge that the formation of NO.sub.X in a tangentially fired furnace can frequently be minimized by judicious control of the air introduced above the fuel-rich main burner zone--i.e. the introduction of so-called overfire air. Judicious control of the overfire air in such circumstances is characterized by the introduction of the overfire air in a manner which supports the formation of the swirling fireball in the furnace while also supporting the sub-stoichiometric conditions in the main burner zone. With regard to supporting the sub-stoichiometric conditions in the main burner zone, it can be appreciated that any increase in the residence time of the fuel in the sub-stoichiometric (fuel rich) main burner zone will further promote the reduction of NO.sub.X.
Notwithstanding the fact that over the years there have been different approaches disclosed in the prior art targeted at the reduction of emissions of the NO.sub.X that is created as a consequence of the combustion of fossil fuels in furnaces, a need still exists in the prior art to improve upon what has been accomplished in the pursuance of these different approaches. For example, the need still exists for an approach which would permit the introduction of overfire air in a manner which promotes a longer residence time of fuel in the sub-stoichiometric conditions of the main burner zone of a tangentially fired furnace while at the same time minimizing the energy required to accomplish an introduction of overfire air in this manner.